Interactions of copper, lead and cadmium with the dissolved, colloidal and particulate components of estuarine and coastal waters

The metal-binding properties of waters collected in the estuarine and coastal environment of central southern England were measured at four-monthly intervals over a one-year period. For Cu and Pb, the metal-binding status of these waters was characterized by differential pulse anodic stripping voltammetry (DPASV) using the combined results of titrations with a competing ligand (ethylenediamine) and titrations with the metal of interest. For Cd, only the latter mode of DPASV-monitored titration provided information on its chemical speciation. Overall, the results suggest that there are three classes of natural Cu-binding ligands and two classes of Pb-binding ligands producing 97–99.99% complexation of Cu and 36–98% complexation of Pb, respectively. Only one class of Cd-binding ligands, responsible for 20–80% complexation, was detected. pCu (pCu = −log [Cu2+]) showed a 2-decade increase from winter to summer in the coastal zone, with a concurrent 1-decade decrease of pPb but no change in pCd. The analytical scheme was applied to the untreated (U), filtered (F) and cross-flow filtered fractions of each sample, so that it was possible to assess the relative role of particulate (> 0.4 μm), colloidal and dissolved (< 3000 Da) components as chemical reactants towards the metals. Although the pool of particulate plus colloidal Cu comprises 30–70% of total Cu, only a fraction reacts to solution composition changes in a manner dictated by chemical equilibrium. Thus Cu reacts mostly with the “truly dissolved” component. Cadmium interacts with the colloidal component as well, while Pb relies heavily on both colloids and particles for control of its dissolved inorganic fraction. There is a close similarity in the values of the conditional stability constants obtained in two — and often all three — successive fractions (U, F and CFF) of a sample, possibly indicating the existence of a limited number of metal-binding agents which can be traced over a wide size range of particles and/or molecules.